Reduction of Barrier Resistance X Area (RA) Product and Protection of Perpendicular Magnetic Anisotropy (PMA) for Magnetic Device Applications

ABSTRACT

A method of forming a MTJ with a tunnel barrier having a high tunneling magnetoresistance ratio, and low resistance x area value is disclosed. The method preserves perpendicular magnetic anisotropy in bottom and top magnetic layers that adjoin bottom and top surfaces of the tunnel barrier. A key feature is a passive oxidation step of a first Mg layer that is deposited on the bottom magnetic layer wherein a maximum oxygen pressure is 10 −5  torr. A bottom portion of the first Mg layer remains unoxidized thereby protecting the bottom magnetic layer from substantial oxidation during subsequent oxidation and anneal processes that are employed to complete the fabrication of the tunnel barrier and MTJ. An uppermost Mg layer may be formed as the top layer in the tunnel barrier stack before a top magnetic layer is deposited.

RELATED PATENT APPLICATIONS

This application is related to the following: U.S. Pat. No. 8,557,407;U.S. Pat. No. 8,592,927; U.S. Pat. No. 8,609,262; US 2012/0205758; andUS 2013/0175644; assigned to a common assignee and herein incorporatedby reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to high performance Magnetic TunnelingJunction (MTJ) elements that include an oxide based tunnel barrierand/or oxide cap layer such as MgO, and in particular, to a method offorming the oxide layer to provide a low resistance x area (RA) productfor good writability and reliability, and to protect (maintain)interfacial perpendicular anisotropy at interfaces with adjoiningmagnetic layers.

BACKGROUND

Magnetoresistive Random Access Memory (MRAM) is based on the integrationof silicon CMOS with MTJ technology, and is a major emerging technologythat is highly competitive with existing semiconductor memories such asSRAM, DRAM, and Flash. Spin-transfer (spin torque) magnetizationswitching described by C. Slonczewski in “Current driven excitation ofmagnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), isimportant due to its potential application for spintronic devices suchas Spin-Torque MRAM on a gigabit scale.

Both field-MRAM and Spin-Torque MRAM have a MTJ element based on atunneling magneto-resistance (TMR) effect wherein a stack of layers hasa configuration in which two magnetic layers called the reference layerand free layer are separated by a thin non-magnetic dielectric layerthat is called a tunnel barrier layer, or more simply, the tunnelbarrier. The MTJ element is typically formed between a bottom electrodesuch as a first conductive line and a top electrode which is a secondconductive line at locations where the top electrode crosses over thebottom electrode.

In various designs of a Spin-Torque MRAM wherein one or both of thereference layer and free layer has perpendicular magnetic anisotropy(PMA), the tunnel barrier contributes to the write function bygenerating spin polarized current. A low RA product is needed for thetunnel barrier for good writability and good reliability. Theoretically,the current density needed to write one MRAM device depends only on thefree layer properties. Therefore, lowering the RA value in the tunnelbarrier means less voltage is required for writing. In addition, thewrite voltage is proportional to the stress applied on the tunnelbarrier during a write operation. Too much stress will lead to anendurance problem, for example, that will affect the reliability ofwriting the same device multiple times without damaging the tunnelbarrier. It is commonly believed that although the strength of thetunnel barrier against applied voltages is reduced as RA decreases, thewriting voltage is reduced even faster. As a result, one will improvewriting reliability by reducing the tunnel barrier RA value. A welloxidized interface between the free layer and tunnel barrier ispreferred to enhance PMA in the free layer. An oxidized cap layer on thefree layer may further enhance PMA along a second interface with thefree layer.

When a spin-polarized current transverses a magnetic multilayer in acurrent perpendicular-to-plane (CPP) configuration, the spin angularmoment of electrons incident on a magnetic layer interacts with magneticmoments of the magnetic layer near the interface between the magneticlayer and non-magnetic spacer. Through this interaction, the electronstransfer a portion of their angular momentum to the magnetic layer. As aresult, spin-polarized current can switch the magnetization direction ofthe free layer if the current density is sufficiently high, and if thedimensions of the multilayer are small. The difference between aSpin-Torque MRAM and a conventional MRAM is only in the write operationmechanism. The read mechanism is the same.

An important consideration when fabricating a tunnel barrier is theoxidation process employed to convert a metal layer into an oxidewithout creating cracks through which metal ions can easily migrate. Thetunnel barrier is typically formed by deposition and oxidation of a thinMg layer, and is required to have a low RA for good reliability. PMA inone or both of the free layer and reference layer must be maintained foroptimum Spin-Torque MRAM performance. The origin of PMA is from theinterface between the magnetic layer and the tunnel barrier whereelectron orbits at the interface have less symmetry than theircounterparts in the bulk of the magnetic layer. Orbits that mostlymaintain in the plane of the interface are energetically favorable,resulting in PMA in the magnetic layer. When oxidation conditions duringthe formation of the tunnel barrier are too strong, a magnetic layerthat interfaces with the tunnel barrier may become partially oxidized,causing a loss of PMA. Thus, the oxidation process must be cleverlydesigned and carefully controlled to preserve PMA in adjoining magneticlayers.

For Spin-Torque MRAM applications, an ultra small MTJ element alsoreferred to as a nanomagnet must exhibit a high TMR ratio or dR/R ofabout 100% or higher at low resistance x area (RA) values of less than20 ohm-μm². Note that dR is the maximum change in resistance in a MTJand R is the minimum resistance of the MTJ. In many cases, MgO ispreferred as the tunnel barrier layer since it provides a higher MRvalue than other oxides. Improvements in tunnel barrier layer qualityare still needed in order to preserve PMA while optimizing RA and TMRratio in the device for Spin-Torque MRAM to be viable in the 90 nmtechnology node and beyond.

SUMMARY

One objective of the present disclosure is to provide a MTJ element thatis able to satisfy design requirements for advanced MRAM and Spin-TorqueMRAM devices wherein substantial PMA in one or both magnetic layers arerequired along with low RA of ≦20 ohm-μm², and a dR/R greater than 100%.

A second objective of the present disclosure is to provide a method forforming the MTJ in the first objective wherein the tunnel barrier isfabricated by an oxidation method that minimizes or prevents oxidationin the adjoining magnetic layers thereby preserving PMA therein andenabling a high TMR ratio.

According to one embodiment, these objectives are achieved by formationof a MTJ element wherein a stack includes a first or bottom magneticlayer and a second or upper magnetic layer that are separated by atunnel barrier comprised substantially of a metal oxide. The metal oxidemay be MgO, or other metal oxides used in the art, or may be alamination of one or more different metal oxides. Moreover, the bottommagnetic layer may be a reference layer in a bottom spin valveconfiguration, a free layer in a top spin valve configuration, oranother functional layer such as a polarizing layer in a three terminalspin-transfer switching device. Furthermore, one or both of the bottomand top magnetic layers may be part of a synthetic antiferromagnetic(SAF) multilayer which contains two magnetic layersantiferromagnetically coupled across a non-magnetic layer (typically Ru)through Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. The metaloxide fabrication may comprise one or more oxidation steps wherein afirst step is a passive oxidation with an oxygen pressure of 10⁻⁵ torror less that is substantially weaker than subsequent oxidation stepswhere oxygen pressure is typically 10⁻³ torr or greater. In one aspectwhere the metal oxide is MgO, a composite tunnel barrier with a Mg/MgOconfiguration is formed after the one or more oxidation steps. The lowerMg layer contacts the bottom magnetic layer and has a substantiallysmaller thickness than the overlying MgO layer. Oxidation stepsfollowing the initial passive oxidation may involve conventionaloxidation processes including but not limited to sequential depositionof one or more metal layers wherein each metal deposition is followed byan oxidation in which the oxygen pressure is typically about 10⁻³ torror greater, or at least 10 to 100 times higher than in the initialpassive oxidation step. Thus, a lower portion of the MgO layer is formedby passive oxidation while an upper portion is formed by one or moreconventional methods such as natural oxidation (NOX), or by directdeposition such as radio frequency (RF) sputtering of MgO which mayrequire no further oxidation steps.

The metal oxide may have a laminated oxide structure wherein twodifferent metals or alloys (M₁, M₂) are employed such that the tunnelbarrier has a M₁/M₁Ox/M₂Ox configuration where a first metal oxide(M₁Ox) is formed by a passive oxidation method and M₂Ox is a secondmetal oxide that is formed by direct deposition or by one or moreconventional oxidation steps having an oxygen pressure of 10⁻³ torr orgreater. M₁ and M₂ are selected from Mg, MgZn, Zn, Al, Ti, AITi, CoMg,Ta, MgTa, Hf, and Zr. After annealing, the tunnel barrier has aM₁Ox/M₂Ox configuration.

The present disclosure also anticipates a dual spin valve design havinga reference layer 1/tunnel barrier 1/free layer/tunnel barrier2/reference layer 2 stack or a free layer 1/tunnel barrier 1/referencelayer/tunnel barrier 2/free layer 2 stack where both tunnel barriers aremade by an oxidation process as described herein. One or both tunnelbarriers may be MgO, MgZnO, ZnO, AIOx, TiOx, AlTiOx, CoMgO, TaOx,MgTaOx, HfOx, or ZrOx, or one or both tunnel barriers have a laminatedoxide structure as explained above.

After the MTJ stack is completed by depositing the top magnetic layer onthe tunnel barrier and then depositing one or more overlying layers suchas a capping layer, an anneal process is used to promote a high TMRratio. Under certain annealing conditions, oxygen in the one or moremetal oxide layers of the tunnel barrier may diffuse into the lower Mglayer to form a substantially uniform tunnel barrier where a metal oxidethat is preferably MgO interfaces with the bottom magnetic layer and thetop magnetic layer to promote PMA in the adjoining magnetic layers forgreater thermal stability. The lower Mg layer serves as a buffer tolimit the amount of oxygen reaching the bottom magnetic layer so thatundesired oxidation is avoided. In other words, the lower Mg layer, orM1 layer in an alternative embodiment, prevents the bottom magneticlayer from being oxidized to an extent that PMA is degraded. Inaddition, higher TMR ratio is realized.

In a preferred embodiment that relates to fabricating a tunnel barrierin a bottom spin valve MTJ configuration, a first Mg layer about 1 to 6Angstroms thick is deposited on a top surface of the reference layer.The reference layer may have intrinsic PMA that is enhanced by contactwith an appropriate seed layer along a bottom surface of the referencelayer. Then a passive oxidation comprised of an oxygen pressure of 10⁻⁵torr or less is applied to oxidize an upper portion of the first Mglayer while a bottom portion of the Mg layer along an interface with thereference layer remains unoxidized. Thereafter, the tunnel barrierformation process continues with one or more conventional oxidationsteps. In one embodiment, a second Mg layer is deposited on thepartially oxidized first Mg layer. The second Mg layer is essentiallycompletely oxidized by a second oxidation process such as a naturaloxidation (NOX) involving an oxygen pressure of 10⁻³ torr or higher.Thus, the second oxidation involves substantially stronger oxidationconditions than the first passive oxidation. Conditions for the NOX stepare selected so that oxygen does not penetrate into the weakly oxidizedMg layer and cause further oxidation therein. A third Mg layer may bedeposited on the second oxidized Mg layer. Next, a free layer isdeposited on the third Mg layer or on the oxidized second metal oxidelayer followed by one or more layers such as a capping layer to completethe MTJ stack. Finally, an anneal process is performed by heating theMTJ stack at a temperature up to 450° C. for a duration up to 90minutes. As a result, the first and third Mg layers absorb oxygen fromthe adjoining second oxidized Mg layer to form a MgO tunnel barrier.

In a second embodiment, a third Mg layer is deposited on the secondoxidized Mg layer and a third oxidation process is performed to form athird oxidized Mg layer before sequentially depositing a fourth Mglayer, depositing the free layer and one or more overlying layers, andapplying an annealing process to form a reference layer/MgO tunnelbarrier/free layer configuration and complete the formation of the MTJstack. In yet another embodiment, the fourth Mg layer in the secondembodiment is treated with a fourth oxidation process to form a fourthoxidized Mg layer. Then a fifth Mg layer, free layer and one or moreoverlying layers are deposited, and an annealing step is performed toyield a MgO tunnel barrier. In all oxidations after the initial passiveoxidation step, oxygen pressure is at least 100 times higher than in thefirst oxidation step. Optionally, a MgO or metal oxide layer may bedeposited by RF sputtering as the uppermost layer in the tunnel barrierstack.

The MTJ stack is an improvement over the prior art since oxidation ofthe bottom magnetic layer that is a reference layer in a bottom spinvalve is minimized or avoided such that PMA is preserved in the bottommagnetic layer. In other words, PMA is enhanced compared with prior artMTJ structures where overoxidation of the bottom magnetic layer causes aloss of PMA. The benefits of enhanced PMA in the bottom magnetic layerare higher TMR ratio and a reduction in RA which leads to better writingperformance and reliability.

The present disclosure also encompasses a spin torque oscillator (STO)structure wherein PMA in a spin polarization layer is preserved andenhanced by an adjoining metal oxide layer that is formed by anoxidation process comprising a passive oxidation as defined herein.

In another embodiment relating to a three terminal device where read andwrite circuits are separated by placing a conductive layer between a STOstack and a RF generator, a tunnel barrier made by an embodiment of thepresent disclosure may be used in the RF generator stack of layers.

In yet another embodiment relating to a three terminal spin-transferswitching device where the read and write circuits are separated throughthe electrical terminals on a polarizing layer, free layer, andreference layer, an oxidation process according to an embodiment of thepresent disclosure may be used to fabricate the tunnel barrier betweenthe free layer and reference layer in the read circuit, and the low RAtunnel barrier between the free layer and polarizing layer in the writecircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a conventional oxidation methodto form a metal oxide tunnel barrier layer where oxygen pressure andprocess time are large enough to cause cracks that extend to anunderlying magnetic layer.

FIG. 2 is a cross-sectional view depicting the result of a passiveoxidation process where a thin metal layer is preserved at an interfacewith a bottom magnetic layer in a bottom spin valve configuration afteran initial tunnel barrier oxidation step according to a first embodimentof the present disclosure.

FIG. 3 is a cross-sectional view showing the deposition of a secondmetal layer on the metal/metal oxide tunnel barrier stack formed in FIG.2.

FIG. 4 is a cross-sectional view of the tunnel barrier in FIG. 3 after asecond oxidation process is used to oxidize the second metal layeraccording to an embodiment of the present disclosure.

FIG. 5 is a cross-sectional view of the tunnel barrier in FIG. 4 after athird metal layer is deposited on the oxidized second metal layer.

FIG. 6 is a cross-sectional view of the composite tunnel barrier in FIG.4 after one or more metal layers are deposited and oxidized by one ormore oxidation methods on the partially oxidized first metal layeraccording to an embodiment of the present disclosure.

FIG. 7 is a cross-sectional view of a tunnel barrier according to thepresent disclosure after the deposition of an uppermost metal layer onthe tunnel barrier structure shown in FIG. 6.

FIG. 8 is a cross-sectional view depicting a free layer formed on thetunnel barrier in FIG. 7 according to an embodiment of the presentdisclosure.

FIG. 9 is a cross-sectional view of the bottom magnetic layer/tunnelbarrier/top magnetic layer stack in a bottom spin valve structurefollowing an anneal process.

FIG. 10 is a cross-sectional view depicting a MTJ nanopillar having abottom spin valve configuration according to an embodiment of thepresent disclosure.

FIG. 11 is a cross-sectional view depicting the passive oxidationprocess on a bottom magnetic layer in a top spin valve configurationaccording to another embodiment of the present disclosure.

FIG. 12 is a cross-sectional view of a partially formed MTJ wherein ametal/metal oxide stack and an upper magnetic layer are formed on abottom magnetic layer in a top spin valve configuration.

FIG. 13 is a cross-sectional view of the bottom magnetic layer/tunnelbarrier/top magnetic layer stack in a top spin valve configurationfollowing an anneal process.

FIG. 14 is a cross-sectional view showing a MTJ nanopillar having a topspin valve configuration according to an embodiment of the presentdisclosure.

FIG. 15 shows a cross-sectional view of a dual spin valve structurewherein one or both tunnel barriers are formed by a process of thepresent disclosure.

FIG. 16 is a cross-sectional view of a bottom magnetic layer/tunnelbarrier/top magnetic layer stack where the tunnel barrier has aM₁/M₁Ox/M₂Ox configuration according to an embodiment of the presentdisclosure.

FIG. 17 is a cross-sectional view of a bottom magnetic layer/tunnelbarrier/top magnetic layer stack where the tunnel barrier has aM₁Ox/M₂Ox configuration following an anneal step according to anembodiment of the present disclosure.

FIG. 18 is a cross-sectional view of a STO device wherein a metal oxidelayer is formed by an oxidation process according to an embodimentdescribed herein.

FIG. 19 is a cross-sectional view of a three terminal device wherein atunnel barrier of a MR sensor component is formed according to anoxidation process of the present disclosure.

FIGS. 20-21 are embodiments of a three terminal spin-transfer switchingdevice where one or both of the tunnel barrier and low RA tunnel barrierare formed according to an oxidation process of the present disclosure.

DETAILED DESCRIPTION

The present disclosure is a method of forming a high performance MTJelement for an ultra high density MRAM, Spin-Torque MRAM, or Spin TorqueOscillator (STO) device wherein RA is reduced, and PMA is betterpreserved in one or both of a free layer and reference layer byemploying a tunnel barrier formation process that prevents substantialoxidation of the free layer and reference layer. Meanwhile, a first(oxide/reference layer) interface and a second (oxide/free layer)interface are used to generate interfacial perpendicular anisotropy andenhance PMA in the adjoining magnetic layers.

As magnetic devices require higher areal density, MTJ elements tend tobecome smaller with shrinking in-plane dimensions and thicknesses forlayers including the reference/pinned layer, tunnel barrier, and freelayer. Control of the tunnel barrier oxidation process is especiallycritical in order to generate a uniform tunnel barrier with low RA inperpendicularly magnetized MTJ devices. In the prior art, formation ofMgO and other tunnel barrier oxides has been accomplished with a singleoxidation or with multiple oxidation steps applied in a sequentialmanner to a plurality of Mg layers, or by direct deposition of a metaloxide (MgO) layer. Typically, the oxidation steps involve oxidationconditions with oxygen pressure greater than 10⁻³ torr in order toachieve the desired RA and TMR ratio. However, prior art MgO fabricationis not compatible with next generation MTJ devices where the magneticlayers at top and bottom surfaces of the tunnel barrier preferably havePMA in order to promote higher thermal stability while maintaining ahigh TMR ratio. In particular, CoFeB layers in a CoFeB/MgO/CoFeBreference layer/tunnel barrier/free layer design that were previously inthe 20-30 Angstrom thickness range are now approaching 10 Angstromsthick or less in order to improve the PMA properties therein.Accordingly, it becomes necessary to develop an improved MgO fabricationprocess that is compatible with the new reference layer and free layerdesign requirements. A higher degree of control must be incorporated inthe MgO (or metal oxide) fabrication to avoid or minimize oxygenincursion into the adjoining magnetic layers while reducing the numberof cracks in the metal oxide layer that might degrade properties such asthe RA value.

As shown in FIG. 1, conventional oxidation 12 of a Mg layer 11 that isdeposited on a reference layer 10 generates cracks 13 in the resultingMgO barrier 11 a such that a certain number of cracks extend to aninterface 14 with the reference layer. The cracks are formed due to thefact that MgO has a smaller lattice than Mg. Cracks allow oxygendiffusion during a subsequent oxidation process into a CoFeB magneticlayer, for example, that lead to undesirable RA and TMR ratio.

We have discovered an improved tunnel barrier process that may beapplied to the formation of MgO or related oxides such as AIOx, MgTaOx,TiO, ZnO, and native CoFeB oxide. The key aspect is to insert two extrasteps comprising a thin metal (Mg) layer deposition followed by apassive oxidation involving an oxygen pressure of 10⁻⁵ torr or less.Thereafter, one or more metal layers are deposited and each depositionis followed by a conventional oxidation having an oxygen pressure of atleast 10⁻³ torr. Passive oxidation as defined herein means that thekinetic energy of the oxygen atoms in the flow is essentially at theminimum level that is reproducible in a manufacturing environment.Typically, the pressure of the oxygen flow is less than 10⁻⁵ torr, andpreferably about 10⁻⁶ torr during passive oxidation. In conventionaloxidation methods, the pressure of oxygen flow is usually above 10⁻³torr, and at least 10 to 100 times greater than in passive oxidation.The unusually weak oxidation condition applied in the passive oxidationstep is employed to minimize the extent of oxidation of the thin metallayer to prevent oxygen diffusion into an underlying (bottom) magneticlayer and to avoid cracks that extend through the thin metal layer to atop surface of the bottom magnetic layer.

According to a preferred embodiment wherein the thin metal layer is Mg,only a top surface of the Mg layer is gently oxidized by passiveoxidation to form a first MgO layer whereas the bottom portion of the Mglayer and the bottom magnetic layer remain unoxidized. Moreover, thegently oxidized first MgO layer will prevent oxygen during subsequentoxidation steps from causing a substantial amount of oxidation in thebottom magnetic layer, even when standard oxidation conditionscomprising ≧10 ⁻³ torr oxygen pressures are employed to oxidizesubsequently deposited metal layers to complete the tunnel barrierformation. One or more additional metal oxide layers may be formed onthe oxidized upper portion of the first metal (Mg) layer. The one ormore additional metal oxide layers may be formed by (a) directdeposition of a metal oxide by a conventional method such as sputteringa metal oxide target, or by (b) depositing a metal layer and thenoxidizing with a process comprising an oxygen pressure of at least 10⁻³torr. It should be understood that when two or more metal oxide layersare formed on a top surface of the oxidized portion of the first metallayer, any combination or repetition of steps (a) and (b) above may beused to form a plurality of metal oxide layers. However, passiveoxidation is a key discovery that enables PMA of the bottom magneticlayer to be better preserved than in prior art tunnel barrierfabrications where only conventional methods are used to form one ormore metal oxide layers.

In a first embodiment depicted in FIGS. 2-6, a series of process stepsare employed to fabricate a tunnel barrier and begin with deposition ofa first metal layer on a bottom magnetic layer followed by a passiveoxidation. Then, a second metal layer is deposited and is oxidized by aconventional oxidation method. A third (upper) metal layer may bedeposited on the oxidized second metal layer. In the exemplaryembodiments, Mg is used as the metal for tunnel barrier fabrication.However, one or more other metals including Al, Ta, Zn, Ti, and Sn, maybe selected instead of Mg. For example, the first (lower) metal layerand upper metal layer may be comprised of a first metal or alloy whileone or more intermediate metal layers may be selected from a secondmetal or alloy unequal to the first metal or alloy. However, for thepurpose of improving throughput, all metal layers are preferablyselected from the same metal or alloy.

Referring to FIG. 2 that relates to a MTJ having a bottom spin valveconfiguration, a bottom magnetic layer that is reference layer 10 isprovided and may be Co, CoFeB, or another alloy comprising two or moreof Co, Fe, Ni, and B and deposited on a seed layer 8 (FIG. 10).Furthermore, the bottom magnetic layer may be a composite with a lowerlaminated stack of layers such as (Co/Ni)_(n) in which n is a laminationnumber. The laminated stack is preferably formed on a seed layer andthere may be an upper CoFeB layer (not shown) to give a(Co/Ni)_(n)/CoFeB configuration, for example. However, an (A1/A2)_(n)laminate may be selected rather than (Co/Ni)_(n). Al may be Co, CoFe, ora CoFeR alloy where R is one of Ru, Rh, Pd, Ti, Zr, Hf, Ni, Cr, Mg, Mn,or Cu. A2 may be Ni, NiCo, NiFe, Pt or Pd. Preferably, Co, CoFeB, or thealloy that is the uppermost layer in the bottom magnetic layer is lessthan about 20 Angstroms thick to enable intrinsic perpendicular magneticanisotropy (PMA) that is enhanced when forming an interface with MgO ora metal oxide tunnel barrier in a later step.

In an alternative embodiment relating to a top spin valve configurationin FIG. 12, the reference layer 10 becomes the top magnetic layer whilethe free layer 30 is the bottom magnetic layer. In this case, the topmagnetic layer may have a CoFeB/(Co/Ni)_(n) or CoFeB/(A1/A2)_(n)configuration where the CoFeB layer contacts a top surface of tunnelbarrier 20.

Returning to FIG. 2, a first metal layer 21 such as Mg with a thicknessbetween 1 and 6 Angstroms is deposited by a sputter deposition method onthe bottom magnetic layer. A Mg film with a thickness less than 1Angstrom is likely to be discontinuous and comprise gaps betweenadjacent grains that extend vertically through the entire film. On theother hand, a Mg layer that is thicker than 6 Angstroms may not becomefully oxidized during a subsequent oxidation and/or anneal step whichmeans degraded interfacial perpendicular anisotropy along the interface14 with the bottom magnetic layer due to the absence of a metaloxide/magnetic layer interface.

A critical feature of the tunnel barrier fabrication sequence asdisclosed herein is a passive oxidation step 16 that is performed totransform an upper portion of Mg layer 21 into MgO while a lower portionof the Mg layer remains unoxidized with no cracks. The upper MgO layer21 a is advantageously used to prevent oxygen during later conventionaloxidation steps with oxygen pressure ≧10⁻³ torr from penetrating Mglayer 21. As indicated previously, pressure of the oxygen flow in aconventional oxidation process is generally a factor of at least 10, andpreferably, about 100 greater in magnitude than employed during ourpassive oxidation of the first Mg layer. The extremely weak oxidationcondition with a maximum oxygen pressure of 10⁻⁵ torr and preferably10⁻⁶ torr for a maximum duration of 1000 seconds guarantees that only anupper portion of the first Mg layer is oxidized and no cracks are formedthrough the first Mg layer. Maximum oxygen pressure is determined bycontrolling oxygen pressure in a closed chamber, or by controlling theoxygen flow rate in a vented chamber. Note that there are oxidizedindentations 15 in oxidized layer 21 a but they do not touch interface14.

Referring to FIG. 3, the following step involves the deposition of asecond Mg layer 22 on oxidized layer 21 a. The second Mg layer and allsubsequent Mg layers have a minimum thickness of 1 Angstrom to yield acontinuous film. The maximum thickness for the second Mg layer dependson the desired RA value for the tunnel barrier, the number of Mg layersdeposited during tunnel barrier fabrication, and the oxidation conditionemployed during the second oxidation step also referred to as the firstconventional oxidation process shown in FIG. 4. In general, RA tends tobecome larger with an increasing number of Mg layers that are oxidizedwith a conventional oxidation process.

In FIG. 4, a first conventional oxidation process 17 that may be anatural oxidation (NOX), for example, is performed using conditions thatcompletely convert the second Mg layer into an oxide layer. The oxidizedsecond Mg layer and the oxidized portion of the first Mg layer form MgOlayer 21 b. Preferably, the oxygen flow rate during the firstconventional oxidation process is at least 1 sccm for a period of 10seconds, an oxidation condition that is considered moderate incomparison to the weak passive oxidation 16. Optionally, a relativelystrong oxidation condition may be used where a flow rate of >1 sccm isapplied for a period up to 600 seconds or with a pressure in the rangeof 0.1 mtorr to 1 torr. However, the first conventional oxidationprocess should not generate cracks that extend to first Mg layer 21 orallow oxygen to diffuse through MgO layer 21 a to further oxidize thefirst Mg layer. The second Mg layer reduces the energy of oxygen thatpasses through layer 22 during the first conventional oxidation process,and MgO layer 21 a further prevents the NOX oxygen from reaching thefirst Mg layer 21. The integrity of an oxidized metal layer may bedetermined by transmission electron microscopy (TEM) analysis to confirmwhether or not cracks are created by a particular oxidation conditionthat may be too extreme for a certain Mg thickness. In other words,cracking is observed by TEM analysis if oxidation conditions are toostrong.

According to one embodiment depicted in FIG. 5 that corresponds to atunnel barrier fabrication sequence involving the fewest number ofoxidation steps according to the present disclosure, an uppermost Mglayer 23 is deposited on MgO layer 21 b to form a composite tunnelbarrier layer 20 having a Mg/MgO/Mg configuration. As described in alater section, an anneal step (not shown) may be used followingcompletion of the MTJ stack shown in FIG. 8 to cause diffusion of oxygenfrom the middle MgO layer 21 b (or MgO layer 21 c in an alternativeembodiment where a plurality of conventional oxidation processes isperformed) into Mg layers 21, 23 thereby forming a MgO layer 21 d whichcontacts both of bottom magnetic layer 10 and top magnetic layer 30(FIG. 9). Optionally, a passive oxidation is followed by a directdeposition of a MgO layer to give a tunnel barrier formation processwith the fewest number of oxidation steps.

In a second embodiment shown in FIG. 6, the sequence illustrated inFIGS. 3-4 may be repeated one or more times to give a MgO layer 21 chaving a thickness t2 on the unoxidized portion of Mg layer 21 that hasa thickness t1. Thus, the MgO layer in FIG. 6 may result from a secondMg layer deposition followed by a first conventional oxidation process,and then a third Mg layer deposition (not shown) followed by a secondconventional oxidation process. The second conventional oxidation mayalso comprise NOX conditions described previously. In the exemplaryembodiment, t2>t1, but the present disclosure also anticipates astructure where t1>t2 since it is well known that MgO has a smallerlattice size than that of Mg. For instance, in an embodiment where thefirst Mg layer has a 3 to 4 Angstrom thickness, and each of the secondand third Mg layers are about 2 Angstroms thick, oxidation of the secondand third Mg layers may result in a MgO layer 21 c thickness less than 4Angstroms. It should be understood that oxidation pressure and durationused to oxidize the second Mg layer may be unequal to the oxidationpressure and duration for oxidation of the third Mg layer. Furthermore,one or both of the first and second conventional oxidation processes mayinclude more than one step. For example, a conventional oxidationsequence may have a first step with a first oxygen flow rate andpressure, and a second step with a second oxygen flow rate and pressureunequal to the conditions in the first step. It is important that allconventional processes employed during the formation of MgO layer 21 cbe at least 10 to 100 times stronger in terms of oxygen pressurecompared with the passive oxidation of the first Mg layer in order toensure that the second and third Mg layers are completely oxidized.

The intermediate tunnel barrier structure depicted in FIG. 6 alsoencompasses a third embodiment wherein a fourth Mg layer (not shown) isdeposited on the oxidized third Mg layer followed by a thirdconventional oxidation process to oxidize the fourth Mg layer.Thereafter, an uppermost (fifth) Mg layer (not shown) may be depositedand remains unoxidized until an optional anneal process after the MTJstack of layers is complete. As indicated earlier, the number of Mglayers that are deposited and oxidized by a conventional oxidationprocess, and the thickness of each Mg layer may be adjusted to influencethe RA value for the tunnel barrier. An anneal process with atemperature up to 450° C. for a duration up to 90 minutes may beemployed during or after the deposition of the MTJ stack in any of theaforementioned embodiments.

The present disclosure also encompasses a tunnel barrier fabricationwherein the oxidation sequence in the second embodiment is modified suchthat a second passive oxidation (PO) process replaces one of theconventional oxidation processes. Thus, there may be a plurality of POsteps employed during fabrication of the tunnel barrier. In one aspect,a first Mg layer is partially oxidized by a first passive oxidation, asecond Mg layer is oxidized by a first NOX step, and a third Mg layer ispartially oxidized by a second passive oxidation before an uppermost Mglayer is deposited. This oxidation sequence may be represented byPO/NOX/PO. However, the second Mg layer may be partially oxidized by asecond PO process and the third Mg layer may be oxidized by a NOX methodin a PO/PO/NOX scheme before an uppermost Mg layer is deposited andremains unoxidized until a subsequent anneal process. Preferably, atleast one NOX step is retained to ensure that a sufficient amount ofoxygen is contained within the oxidized Mg layers to enable diffusioninto unoxidized portions of Mg layers during the anneal process andthereby forming an essentially uniform MgO tunnel barrier, or metaloxide tunnel barrier in embodiments where the metal is not Mg.

It should be understood that the third embodiment may be modifiedwherein one or more of the NOX steps are replaced by a passiveoxidation. According to one fabrication sequence, a first Mg layer isdeposited and partially oxidized by a first passive oxidation process, asecond Mg layer is deposited and oxidized by a first NOX process, athird Mg layer is deposited and oxidized by a second NOX process, andthen a fourth Mg layer is deposited and partially oxidized by a secondpassive oxidation before the uppermost Mg layer is deposited. Thisoxidation scheme is represented by PO/NOX/NOX/PO. Instead of aPO/NOX/NOX/PO sequence, a series of oxidation steps represented byPO/PO/NOX/PO, PO/PO/PO/NOX, or PO/NOX/PO/PO may be used wherein at leastone oxidation involves a NOX step to ensure a sufficient quantity ofoxygen within the tunnel barrier layer stack to completely oxidize allmetal layers therein following free layer formation and a subsequentanneal process.

The present disclosure also anticipates that the passive oxidationprocess may comprise nitrogen gas so that an upper portion of the firstmetal layer deposited in a tunnel barrier stack becomes a metaloxynitride. The first metal layer preferably has a thickness between 1and 6 Angstroms. In an alternative embodiment, nitrogen in the absenceof oxygen is used to deposit a first metal nitride layer on the firstmetal layer. Typically, a metal nitride is deposited by using ionizednitrogen atoms and Ar to hit a metal target. As a result, the metalnitride is sputter deposited onto a substrate. One can control the flowrate of nitrogen gas to change the ratio between Ar and ionized nitrogenatoms and thereby change the nitrogen content in the metal nitride suchas MgN_(x). This process may be defined as a passive nitridation processif oxygen is excluded and there is a maximum nitrogen pressure of 10⁻⁵torr. As a result, the bottom magnetic layer/first metal layer interfacedoes not react with nitrogen and a first metal/first metal nitride stackis formed. Subsequent layers formed on the metal oxynitride or firstmetal nitride layer may be metal oxide layers fabricated with aconventional oxidation method of a metal layer, or by direct deposition,or one or more of the subsequent layers may have a metal oxynitride ormetal nitride composition. Thereafter, an uppermost metal layer may bedeposited on a top surface of an underlying metal oxide, metaloxynitride, or metal nitride layer. An anneal process at a temperatureup to 450° C. and with a duration up to 90 minutes may be performedduring the uppermost metal deposition or after a top magnetic layer andcapping layer are sequentially formed on the uppermost metal layer.

Referring to FIG. 7, an uppermost Mg layer 23 is deposited on MgO layer21 c once the final conventional oxidation process or second passiveoxidation is completed in the aforementioned embodiments related to FIG.6. The thickness t3 of the final Mg layer is preferably at least 1Angstrom. A maximum thickness for t3 is determined in part by thetemperature and time involved in a subsequent annealing step. Inparticular, t3 should not be so large that oxygen from MgO layer 21 cdoes not diffuse into all portions of Mg layer 23 and fail to oxidize aportion thereof along top surface 23 s. It is important that a metaloxide/top magnetic layer interface be formed in order to maximize PMA inthe top magnetic layer. As mentioned previously, an anneal process maybe employed during deposition of the uppermost metal layer 23. Inaddition, a second anneal process may occur after the MTJ stack iscompleted. As temperature is increased up to 450° C. and/or process timeis lengthened in any of the anneal processes, then oxygen diffuses agreater distance into layer 23. Note that a relatively thick Mg layer 23(t3≧3 Angstroms) will provide a substantial contribution to the final RAvalue. Therefore, t3 is preferably kept between 1 to 3 Angstroms. It isimportant that unoxidized Mg layers 21, 23 are maintained on oppositesurfaces of MgO layer 21 c during subsequent steps related to formationof a top magnetic layer and overlying layers such as a capping layer sothat oxygen does not penetrate and oxidize a portion of the bottommagnetic layer 10 and top magnetic layer 30. Thus, metal layers 21, 23prevent oxidation of adjoining magnetic layers but serve as a pathwayfor the tunnel barrier to become completely oxidized at a later timeafter the MTJ stack is completed.

In FIG. 8 that relates to a bottom spin valve configuration, MTJ stack40 is shown after a free layer 30 is formed as the top magnetic layer onthe uppermost Mg layer 23. In this intermediate structure, a firstinterface 14 has a CoFeB/Mg composition, for example, while a secondinterface 28 may also have an Mg/CoFeB composition. In an alternativeembodiment, the top magnetic layer may be comprised of Fe, or an alloyof two or more of Co, Fe, Ni, and B. The present disclosure alsoencompasses embodiments where the free layer has a syntheticantiferromagnetic (SAF) configuration wherein two ferromagnetic layersare separated and antiferromagnetically coupled that a layer such as Ru.Moreover, the top magnetic layer may have a moment diluting layer suchas Ta or Mg formed between two magnetic layers that areferromagnetically coupled. In yet another embodiment, the free layer maybe a composite with a CoFeB, Co, or CoFe layer that has a bottom surfacealong interface 28, and a laminated stack such as (Co/Ni)_(n), or(A1/A2)_(n) described previously formed on a top surface of the CoFeB,Co, or CoFe layer.

MTJ stack 40 may further comprise a capping layer (not shown) formed ona top surface 30 s of free layer 30. For example, the capping layer mayinclude one or more of Ru and Ta to protect the free layer duringsubsequent process steps such as a chemical mechanical polish processthat produces a smooth top surface on the MTJ stack. In anotherembodiment, the capping layer may be a metal oxide to generateinterfacial perpendicular anisotropy along the top surface 30 s andenhance PMA within the top magnetic layer. According to one aspect ofthe present disclosure, a metal oxide capping layer may be formed byemploying the tunnel barrier formation process disclosed herein. Thus,both of the tunnel barrier layer and capping layer may be MgO, forexample, that has been fabricated by depositing a first Mg layerfollowed by a passive oxidation step. A bottom portion of the firstmetal layer in the capping layer remains unoxidized to prevent oxidationof the free layer. Thereafter, at least a second Mg layer is depositedon the partially oxidized first Mg layer followed by a conventionaloxidation process. The formation of an uppermost Mg layer that is notsubjected to an oxidation process may be omitted during capping layerformation since there is no subsequently deposited magnetic layer thatrequires protection from oxidation. During the subsequent anneal processdescribed previously, oxygen from the oxidized second Mg layer diffusesinto the bottom portion of the capping layer to form a metal oxideinterface with a top surface of the top magnetic layer. In analternative embodiment, the tunnel barrier may be an oxide made of afirst metal or alloy such as MgTaO while the capping layer is made of asecond metal or alloy that is MgO, for example.

Referring to FIG. 9, an anneal process is employed to cause diffusion ofoxygen within MgO layer 21 c (FIG. 8) into adjoining Mg layers. As aresult, MgO layer 21 d is formed that has a bottom surface along firstinterface 14 and a top surface along second interface 28 to enhance PMAwithin bottom magnetic layer 10 and top magnetic layer 30, respectively.The anneal process comprises applying a temperature up to 450° C. for aperiod of up to 90 minutes. Anneal temperatures around 400° C. arepreferred when the resulting MTJ structure is incorporated in a CMOSdevice.

According to an embodiment depicted in FIG. 10, a MTJ nanopillar 40 n isfabricated after the MTJ stack is completed and annealed by following aconventional patterning and etching sequence. In the exemplaryembodiment, the MTJ nanopillar is formed on a substrate 6 such as abottom electrode. The MTJ nanopillar comprises a seed layer 8 on thesubstrate and an uppermost capping layer 35 having a planar top surface35 s. A sidewall 38 extends from the top surface to substrate 6. In thisdrawing, the x-axis and y-axis directions are in the planes of thelayers while a thickness of each MTJ layer is determined along thez-axis direction. The MTJ nanopillar top surface 35 s may have acircular or elliptical shape from a top-down view along the z-axis. Aplurality of MTJ nanopillars is typically arrayed in a design withcolumns and rows on the substrate.

Referring to FIG. 11, the present disclosure also encompasses a methodof forming a tunnel barrier in a MTJ with a top spin valve structure. Inthis process flow, a first metal (Mg) layer 21 is deposited on a bottommagnetic layer that is a free layer 30. Thereafter, a passive oxidationas described earlier is performed to gently oxidize a top portion of thefirst metal layer to form a first metal oxide layer 21 a while a bottomportion of the first metal layer and bottom magnetic layer remainunoxidized.

FIG. 12 depicts a MTJ structure after one or more metal layers aredeposited on metal oxide layer 21 a. Each of the one or more metallayers is oxidized by a conventional oxidation process as describedpreviously to form metal oxide layer 21 c that includes the first metaloxide layer. Optionally, one of the conventional oxidation processes maybe replaced by a second passive oxidation. Then, an uppermost metallayer 23 is deposited. Once the uppermost metal layer in the tunnelbarrier stack is laid down, a top magnetic layer that is a referencelayer 10 is deposited. The top magnetic layer forms an interface 27 withmetal layer 23 while the bottom magnetic layer forms an interface 16with first metal layer 21.

FIG. 13 illustrates the MTJ structure in FIG. 12 after an anneal processis performed. As a result, metal oxide layer 21 d is the tunnel barrierthat has a first interface 16 with the bottom magnetic layer, and asecond interface 27 with the top magnetic layer. PMA in preserved anenhanced in both magnetic layers because of the controlled oxidationprocesses involved in preparing the tunnel barrier, especially thepassive oxidation applied to the first metal layer.

In FIG. 14, one example of a MTJ with a top spin valve structure havinga tunnel barrier 21 d formed according to an embodiment of the presentdisclosure is depicted. All layers are retained from the bottom spinvalve stack in FIG. 10 except that magnetic layers 10 and 30 areswitched.

Another embodiment of the present disclosure is illustrated in FIG. 15where a MTJ nanopillar 70 n has a dual spin valve structure having afirst tunnel barrier 21 d 1 between a first reference layer 10 a andfree layer 30, and a second tunnel barrier 21 d 2 between the free layerand a second reference layer 10 b. Both tunnel barrier layers may befabricated according to a process flow that includes a passive oxidationof a first metal layer as described in one of the previously describedembodiments. Alternatively, the dual spin valve may have a configuration(not shown) represented by FL1/tunnel barrier 1/reference layer/tunnelbarrier 2/FL2 where FL1 is a first free layer and FL2 is a second freelayer. Formation of both tunnel barriers (layers 21 d 1, 21 d 2) mayproceed according to an embodiment described previously related totunnel barrier 21 d. The dual spin valve structure may be fabricated bya sequence wherein a second tunnel barrier 21 d 2 is formed on stackthat has a first magnetic layer 10 a/first tunnel barrier 21 d1/magnetic layer 30 configuration. Then a third magnetic layer 10 b isdeposited on the second tunnel barrier. With regard to tunnel barrier 21d 2, a first metal layer (not shown) is deposited on magnetic layer 30followed by a passive oxidation process with a maximum oxygen pressureof 10⁻⁵ torr for up to 1000 seconds. The passive oxidation processoxidizes an upper portion of the first metal layer while a bottomportion of the first metal layer at an interface with a top surface ofmagnetic layer 30 remains unoxidized. One or more metal oxide layers maythen be formed on the oxidized upper portion of the first metal layeraccording to methods described in previous embodiments. Thereafter, anuppermost metal layer may be deposited on a top surface of the one ormore metal oxide layers before a third magnetic layer 10 b is formed.There may be a capping layer 35 formed on a top surface of magneticlayer 10 b to complete the MTJ stack. An anneal process with atemperature up to 450° C. for up to 90 minutes may be performed duringdeposition of a first metal layer in both tunnel barriers 21 d 1 and 21d 2, or an anneal process may be performed after all layers in the dualspin valve MTJ are formed.

According to another embodiment shown in FIG. 16, a composite tunnelbarrier 50 is formed according to an embodiment where a first metallayer (M₁) 21 is deposited and an upper portion thereof is partiallyoxidized by a passive oxidation to form a first metal oxide layer 21 arepresented by a M₁/M₁Ox configuration. Then, one or more metal layersmade of a second metal (M₂) where M₂ with a different from that of M₁may be deposited and oxidized by one or more conventional oxidationprocesses to form a second metal oxide layer 22 a. Optionally, one ofthe conventional oxidation processes may be replaced by a second passiveoxidation. An uppermost metal layer 23 that may have either a M₁ or M₂composition is deposited on metal oxide layer 22 a.

In FIG. 17, the MTJ structure from FIG. 16 is shown after an anneal isperformed wherein a first metal oxide layer 21 e is formed as a resultof oxygen diffusion into first metal layer 21 and oxidation thereof toyield an essentially uniform M₁Ox layer from the intermediate M₁/M₁Oxstack. There is also a second metal oxide layer 22 b resulting fromoxygen diffusion into uppermost metal layer 23 and oxidation thereof toform an essentially uniform M₂Ox layer from the intermediateM₂Ox/uppermost metal oxide stack. Therefore, a composite tunnel barrier50 is formed wherein at least the first metal oxide layer is fabricatedby a process including a passive oxidation of a metal layer.

The present disclosure also anticipates an embodiment relating to a STOdevice wherein a metal oxide layer made according to a process sequencedisclosed herein adjoins a spin polarization (SP) layer in order topreserve and even enhance PMA in the SP layer. Previously, we discloseda spin torque oscillator (STO) device in U.S. Pat. No. 8,582,240 whereinnon-magnetic layers formed adjacent to a spin polarization layer andoscillation layer may be metal oxides.

Referring to FIG. 18, a MAMR writer based on perpendicular magneticrecording (PMR) is depicted. There is a main pole 81 with a sufficientlylarge local magnetic field to write the media bit 85 in medium bit layer84. Magnetic flux 88 in the main pole proceeds through the air bearingsurface (ABS) 86-86 and into medium bit layer 84 and soft underlayer(SUL) 87. A portion of the flux (not shown) returns to the write headwhere it is collected by write shield 82. For a typical MAMR writer, themagnetic field generated by the main pole itself is not strong enough toflip the magnetization 89 of the medium bit in order to accomplish thewrite process. However, writing becomes possible when assisted by a spintorque oscillator (STO) 83 positioned between the main pole and writeshield.

The STO is comprised of a high moment magnetic layer 90, and a secondmagnetic layer 91 that preferably has perpendicular magnetic anisotropy(PMA). Between layers 82 and 90, 90 and 91, and 91 and 81, there arenonmagnetic layers 92, 93, 94, respectively, to prevent strong magneticcoupling between adjacent magnetic layers. Non-magnetic layer 94 may bea metal oxide layer in order to form a metal oxide/magnetic layerinterface with magnetic layer 91 and thereby preserving or enhancing PMAtherein. Likewise, non-magnetic layer 92 may be a metal oxide layer topreserve or enhance PMA in non-magnetic layer 90.

An external current source 98 creates a bias current across the mainpole and write shield. The applied dc results in a current flow in adirection from lead 101 into oscillation layer (OL) 90 and then throughnon-magnetic layer 93 and into SP layer 91 before exiting through lead100. Direct current generated by source 98 is spin polarized by magneticlayer 91, interacts with magnetic layer 90, and produces a spin transfertorque that causes oscillation with a precession angle 95 in magneticlayer 90 hereafter called the oscillation layer (OL). The large angleoscillatory magnetization of OL 90 generates a radio frequency (1)usually with a magnitude of several to tens of GHz. This rf field (notshown) interacts with magnetization 89 of medium bit 85 and makes themagnetization oscillate into a precessional state 97 thereby reducingthe coercive field of medium bit 85 to allow switching by the main polefield 88.

A key feature of the present disclosure is to provide a metal oxidecomposition in one or both of non-magnetic layers 92, 94 made by apassive oxidation process as disclosed in one of the previousembodiments. As a result, PMA is preserved in an adjoining magneticlayer that is SP 91 or OL 90, respectively. According to one embodiment,layer 94 is a metal oxide formed by depositing a first metal layer onthe main pole layer and then performing a passive oxidation. One or moremetal oxide layers are formed on the upper oxidized portion of the firstmetal layer before an uppermost metal layer is laid down. Then, layers91, 93, 90, and 92 are sequentially formed before the write shield isfabricated. In one aspect, layer 92 is formed by depositing a firstmetal layer on OL 90 and then performing a passive oxidation. Next, oneor more oxide layers are formed on an oxidized upper portion of layer 92before an uppermost metal layer is deposited. An anneal process may beperformed at this point or when each of the uppermost metal layers aredeposited. As a result, oxidation processes to form metal oxide layers92, 94 are well controlled and prevent substantial oxygen incursion intoSP layer 91 and OL layer 90. In an alternative embodiment, the STOlayers may be formed in reverse order on the main pole layer. Otheraspects of previous embodiments are retained including the compositionof metal oxide layers, methods to form one or more oxide layers on theoxidized first metal layer, and an anneal process during deposition ofthe uppermost metal layer or after all STO layers are laid down.

Another embodiment of the present disclosure is related to aperpendicular spin torque oscillator (PSTO) device wherein a highdensity STO current is isolated from a low density RF generation currentthat we previously disclosed in U.S. Pat. No. 8,203,389. In particular,a tunnel barrier in a RF generator portion of the three terminal devicemay be a metal oxide layer such as MgO that is formed by a processdescribed in a previous embodiment.

Referring to FIG. 19, a PSTO device is shown with a STO component 103and a RF generation component 104 hereafter referred to as “RFgenerator” or “MR sensor” that are separated by a non-magneticconductive layer 114. In one aspect, STO 103 is a giant magnetoresistivejunction comprised of a PMA magnetic layer 111 that serves as a magneticreference layer (MRL), and a stack including a first junction layer alsoknown as non-magnetic spacer 112, second PMA magnetic layer 113 a, and asoft magnetic layer 113 b that are sequentially formed on the MRL. PMAlayer 111 and second PMA layer 113 a may be comprised of a (A1/A2)_(n)laminate as described earlier.

Spacer 112 may be made of a conductive material such as Cu, or may havea confining current pathway (CCP) configuration in which Cu pathways areformed in an oxide matrix such as AlO_(x). Layers 113 a, 113 b areexchange coupled to each other and form a composite magnetic oscillationlayer (MOL) wherein the magnetization in each layer is free to oscillatewhen subjected to an applied magnetic field perpendicular to the planesof the layers, and when an electric current of sufficiently high densityflows in a direction perpendicular to the planes of the layers from afirst electrical terminal 122 to a second electrical terminal 121. Thehigh current density is preferably in the range of 1×10⁷ to 1×10⁹Amps/cm² in order to exceed the critical current density for causing aspin torque effect on the MOL. It is believed that reflected electronsfrom the MRL/spacer interface excite the MOL layer and thereby induce anoscillation state in layers 113 a, 113 b with significant in-planeamplitude. Note that PMA layer 113 a has the same oscillation frequencyas soft magnetic layer 113 b but a smaller in-plane magnetizationcomponent. Soft magnetic layer 113 b may be made of CoFe, a CoFe alloy,or a composite thereof.

Non-magnetic conductive layer 114 is preferably a metal made of Cu orthe like, or a metal alloy having a bottom surface that contacts anuppermost layer of STO 103, and with a top surface that adjoins a bottomlayer in RF generator 104. Preferably, conductive layer 114 has a widthin an in-plane direction along the x-axis that is greater than the widthw of the layers in the STO and RF generator in order to allow anelectrical connection to a first electrical terminal hereafter referredto as first terminal 122.

According to one embodiment, RF generator 104 is a magnetoresistive (MR)sensor with a TMR configuration in which a MTJ has a magnetic sensinglayer 125, a second junction layer hereafter referred to as tunnelbarrier 126, reference layer 127, exchange coupling layer 128, pinnedlayer 129, and AFM layer 130 are sequentially formed on a top surface ofconductive layer 114. Optionally, when reference layer 127 has PMA,layers 128-130 may be omitted. An important feature is that magneticsensing layer 125 should have a Mst value within about ±50% of the Mstvalue for MOL layer (113 a, 113 b). Moreover, magnetic sensing layer 125may be a single layer or a composite and is magnetostatically coupled tosoft magnetic layer 113 b such that when an oscillating state isestablished in the MOL, an oscillation state is induced in the sensinglayer with substantially the same frequency as in layers 113 a, 113 b.Preferably, in an embodiment wherein MR sensor 104 and STO 103 haveessentially the same width w, the MR sensor is aligned vertically abovethe STO such that sidewalls 103 s, 104 s are substantially coplanar inorder to provide an efficient magnetostatic coupling between softmagnetic layer 113 b and sensing layer 125. A key aspect is that tunnelbarrier 126 is a metal oxide made by a process including partialoxidation of a first metal layer by a passive oxidation process asdescribed previously to preserve PMA in magnetic sensing layer 125, andin reference layer 127 in an embodiment where layers 128-130 areomitted. As a result, RF generator 104 has lower RA and higher TMR ratiocompared with prior art PSTO devices where the tunnel barrier isfabricated by conventional oxidation processes.

During an operating mode, an external magnetic field 105 is applied tothe entire PSTO structure including STO 103 and RF generator 104 ineither a (+) or (−) y-axis direction to align the perpendicularmagnetization components of MRL 111, MOL 113 a/113 b, and magneticsensing layer 125 in the same direction as the field direction.Preferably, MRL 111 has an entirely perpendicular to plane magnetizationorientation while the MOL and magnetic sensing layer magnetizations aretilted partially out of the film plane. When a high density currentflows from first terminal 122 to second terminal 121, electrons passthrough the MOL layer to MRL 111. A portion of the electrons arereflected from the MRL/spacer 112 interface back into the MOL to excitethe MOL magnetization from a quiescent state into a significant in-planeoscillation. Subsequently, the oscillating in-plane magnetizationcomponent in MOL 113 a/113 b produces an oscillating magnetic field inmagnetic sensing layer 125. The in-plane magnetization oscillation ofthe magnetic sensing layer has a 180 degree phase difference comparedwith that of MOL which means the MOL and magnetic sensing layer are in apseudo anti-ferromagnetic coupled FMR mode. Therefore, with magneticsensing layer 25 being part of MR sensor 104 and a DC current flowingbetween first terminal 122 and third terminal 120 in either direction,an AC voltage signal can be generated between the first and thirdterminals from a resistance change in the MR sensor due to magnetostaticcoupling between the magnetic sensing layer and the oscillating MOL.

According to another embodiment, a three terminal spin-transferswitching device shown in FIG. 20 may comprise a tunnel barrier layerand a low RA barrier formed by using the passive oxidation process andone or more methods such as NOX and RF sputtering described previously.Similar to the three terminal structure we have disclosed in U.S. Pat.No. 7,978,505, three magnetic layers including a polarizing layer, freelayer, and a reference layer are separated by two non-magnetic layers.In FIG. 20, the non-magnetic layer 214 between the free layer 213 andthe reference layer 215 is a tunnel barrier with normal RA to producethe read signal in the read circuit 221, which contains the free layerand its terminal (electrode 202), the normal RA tunnel barrier, thereference layer and its terminal (electrode 203) and the source of readsignal and a detecting setup, a sense amplifier (not shown), forexample. The non-magnetic layer 212 between the free layer and thepolarizing layer 211 may be a metal layer or a low RA tunnel barrier toproduce the spin-transfer torque for switching the free layer in thewrite circuit 220, which contains the free layer and its terminal(electrode 202), the metal spacer or low RA tunnel barrier, thepolarizing layer and its terminal (electrode 201) and the source ofwriting voltage (not shown). The separation of the write and readcircuits ensures that the write circuit, where the writing current canbe much larger than that in a normal two terminal device, has a low RAwhile the read circuit has a normal RA to generate decent read signals.The previously disclosed oxidation process may be used to fabricate thenormal RA barrier 214 to enhance MR ratio and preserve PMA, and may alsobe employed in formation of the low RA barrier 212 to further reduce theRA value.

In an alternative embodiment depicted in FIG. 21, the order of formingthe layers 211-215 in the three terminal device shown in FIG. 20 may bereversed such that the reference layer 215 is formed as the bottommostlayer followed by the tunnel barrier 214, free layer 213, metal or lowRA barrier 212, and a polarizing layer as the uppermost layer. In thiscase, the read circuit 221 comprises a first terminal (electrode 201)attached to the reference layer, a second terminal (electrode 202)connected to the free layer, and layers 213-215 between the first andsecond terminals. Meanwhile, the write circuit 220 comprises the secondterminal, a third terminal (electrode 203) attached to the polarizinglayer, and layers 211-213 between the second and third terminals.

To demonstrate the effectiveness of the tunnel barrier fabricationmethod of the present invention, an experiment was performed to buildMTJ nanopillars in 10 Mb memory device arrays with a reference layer/MgOtunnel barrier/free layer/cap layer configuration. A current-in-planetunneling (CIPT) technique was used to measure RA of the stack and TMRratio of each MTJ nanopillar. In all examples in Table 1, the bottom andtop magnetic layers that adjoin the tunnel barrier are both CoFeB andthe capping layer is MgO.

A conventional tunnel barrier formation process currently practiced bythe inventors is used for the MTJ shown in row 1 of Table 1 and includesdeposition of three Mg layers with thicknesses of 6.6 Angstroms (firstMg layer), 3 Angstroms (second Mg layer), and 2.5 Angstroms for theuppermost Mg layer. The first Mg layer is oxidized with a NOX processcomprised of a 5 sccm O₂ flow rate for 80 seconds, and the second Mglayer is oxidized with a two part NOX process where the first step has a5 sccm O₂ flow rate for 20 seconds and the second step has a O₂ pressurecontrol of 1 torr for 600 seconds. After the third Mg layer is depositedon the oxidized second Mg layer and a capping layer is formed as theuppermost layer, the complete MTJ is annealed with a process comprising400° C. for 30 minutes that is common to all three MTJ nanopillarstructures.

Row 2 represents MTJ with an MgO tunnel barrier built according to anembodiment of the present disclosure. A key feature is that the first Mglayer with a 3.3 Angstrom thickness is treated with a passive oxidation(PO) method with an oxygen flow pressure <10⁻⁶ torr for 20 seconds. Thena second Mg layer with a 3.3 Angstrom thickness is deposited and a NOXprocess is performed with a 5 sccm O₂ flow rate for 80 seconds. Next, athird Mg layer is deposited and a two part NOX process is appliedwherein a first step comprises a 5 sccm O₂ flow rate for 20 seconds, anda second step has a pressure control at 1 torr for 600 seconds. Finally,a fourth Mg layer with a 2.5 Angstrom thickness is deposited and thestructure is annealed at 400° C. after the MTJ stack is completed.

Row 3 represents a MTJ with an MgO tunnel barrier built according toanother embodiment of the present disclosure. A key feature is that thefirst Mg layer with a 2.75 Angstrom thickness is treated with a passiveoxidation (PO) method with an oxygen flow pressure of <10⁻⁶ torr for 20seconds. Then a second Mg layer with a 2.25 Angstrom thickness isdeposited and a NOX process is performed with a 5 sccm O₂ flow rate for80 seconds. Next, a third Mg layer with a 3 Angstrom thickness isdeposited and a two part NOX process is applied with a 5 sccm, 20 secondO₂ flow for the first step and a 18 sccm, 500 second O₂ flow for thesecond step. Then a fourth Mg layer having a 4.5 Angstrom thickness isdeposited and a third NOX process is performed wherein O₂ flow rate is 5sccm for 20 seconds. Finally, a fifth Mg layer with a 2.5 Angstromthickness is deposited and the structure is annealed at 400° C. afterthe MTJ stack is completed.

In rows 1 and 2 in Table 1, the total thickness of all deposited Mglayers is around 12 Angstroms. There are five separate Mg layers in thethird example (row 3 process) with a combined thickness of 15 Angstromsthat leads to a slightly higher RA value than in row 2 but RA is stilllower than that shown for the MTJ in row 1.

TABLE 1 Magnetic Properties of patterned MTJ nanopillars withCoFeB/MgO/CoFeB/MgO configuration after anneal at 400° C. for 30 min. #MgO tunnel barrier formation process RA TMR % 1 Mg6.6/NOX(5 sccm, 80s)/3Mg/NOX(5 sccm, 20 20 130 s)/NOX(1 torr, 600 s)/Mg2.5 2Mg3.3/PO(<10⁻⁶ torr, 20 s)/3.3Mg/NOX(5 12 140 sccm, 80 s)/3Mg/NOX(5sccm, 20 s)/NOX(1 torr, 600 s)/Mg2.5 3 Mg2.75/PO(<10⁻⁶ torr, 20s)/2.25Mg/NOX(5 17 140 sccm, 80 s)/3Mg/NOX(5 sccm, 20 s + 18 sccm, 500s)/Mg4.5/NOX(5 sccm, 20 s)/Mg2.5

A comparison of MTJ nanopillar in row 2 to the conventional MTJ in row 1clearly indicates several benefits associated with having a passiveoxidation as the initial oxidation step in fabricating a tunnel barrier.In particular, there is an increase in TMR ratio from 130% to 140%, anda decrease in RA from 20 to 12. It is important to note that theincrease in TMR ratio is due to better preservation of PMA in the CoFeBmagnetic layers. The row 3 MTJ nanopillar has a tunnel barrier made witha process that has one additional Mg layer deposition and an extra NOXoxidation compared with the MTJ in row 2 to intentionally produce athicker MgO layer with slightly higher RA. The example in row 3 alsoexhibits a better TMR ratio and lower RA compared with the MTJ in row 1with a conventional MgO tunnel barrier. Thus, we have demonstrated thatthe improved tunnel barrier formation process described herein hasflexibility in fabricating a variety of tunnel barriers.

Table 1 results suggest that the tunnel barrier fabrication of thepresent disclosure enables PMA in the reference layer and free layer tobe maintained or even enhanced as demonstrated by the larger TMR ratio.Moreover, thermal stability of at least 400° C. in MTJ nanopillars whichis required for compatibility with CMOS processes is achieved since allof the desired properties in Table 1 were measured after an annealprocess for 30 minutes at 400° C. An elevated anneal temperature near400° C. is also beneficial in crystallizing amorphous magnetic layerssuch as CoFeB and the MgO tunnel barrier to ensure a higher TMR ratio.

While this disclosure has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this disclosure.

We claim:
 1. A method of forming a magnetic tunnel junction (MTJ) stackof layers including a tunnel barrier layer between two magnetic layers,comprising: (a) providing a bottom magnetic layer with perpendicularmagnetic anisotropy (PMA); (b) depositing a first metal layer that formsa bottom magnetic layer/first metal layer interface; (c) performing afirst passive oxidation process with a maximum oxygen pressure of about10⁻⁵ torr, the first passive oxidation process oxidizes an upper portionof the first metal layer while a bottom portion of the first metal layeralong the bottom magnetic layer/first metal layer interface remainsunoxidized; (d) forming one or more metal or metal oxide layers on theoxidized portion of the first metal layer wherein steps (b)-(d) form atunnel barrier layer; and (e) depositing a top magnetic layer on a topsurface of the tunnel barrier layer.
 2. The method of claim 1 whereinthe first passive oxidation process has a maximum duration of about 1000seconds.
 3. The method of claim 1 wherein the first metal layer has athickness from about 1 to 6 Angstroms.
 4. The method of claim 1 wherethe one or more metal oxide layers formed on the oxidized portion of thefirst metal layer are formed by one or more conventional methodscomprising: (a) direct deposition of a metal oxide layer; (b) depositinga metal layer and then oxidizing all or part of the metal layer with anoxygen pressure that is at least 10⁻³ torr; and (c) any combination orrepetition of steps (a) and (b) above.
 5. The method of claim 1 whereinthe first metal layer, and the one or more metal oxide layers arecomprised of a metal or alloy selected from Mg, Al, Ta, Ti, Zn, Sn,MgZn, AITi, CoMg, and MgTa.
 6. The method of claim 1 wherein the bottommagnetic layer is part of a synthetic antiferromagnetic (SAF) layer andis antiferromagnetically coupled to a second magnetic layer through acoupling layer in a Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. 7.The method of claim 1 wherein the top magnetic layer is part of a SAFlayer and is antiferromagnetically coupled to a second magnetic layerthrough a coupling layer in a Ruderman-Kittel-Kasuya-Yosida (RKKY)interaction.
 8. The method of claim 1 wherein the passive oxidationprocess is further comprised of nitrogen.
 9. The method of claim 1wherein at least one of the one or more metal oxide layers formed on thetop surface of the oxidized portion of the first metal layer is furthercomprised of nitrogen and has a metal oxynitride composition.
 10. Themethod of claim 1 further comprising an anneal process during orfollowing the deposition of the MTJ stack, the anneal process comprisesa temperature up to about 450° C. for a duration up to about 90 minutes.11. The method of claim 1 wherein the first metal layer is comprised ofa different metal than a metal in the one or more metal or metal oxidelayers.
 12. The method of claim 1 further comprised of forming a cappinglayer on the top magnetic layer.
 13. The method of claim 12 wherein thecapping layer is a metal oxide layer that is formed by a processsequence comprising one or more of a direct deposition method, a passiveoxidation process, or an oxidation process comprising an oxygen pressureof at least 10⁻³ torr.
 14. The method of claim 1 further comprising theformation of a dual spin valve MTJ stack by a process comprising: (a)forming a second stack of layers on a top surface of the top magneticlayer, the second stack is formed by a process comprising; (1)depositing a second metal layer that contacts a top surface of the topmagnetic layer; (2) performing a second passive oxidation process with amaximum oxygen pressure of about 10⁻⁵ torr, the second passive oxidationprocess oxidizes an upper portion of the second metal layer while abottom portion of the second metal layer at an interface with the topmagnetic layer remains unoxidized; and (3) forming one or more metal ormetal oxide layers on the oxidized portion of the second metal layer,steps (1)-(3) form a second tunnel barrier; and (b) depositing a thirdmagnetic layer on a top surface of the second tunnel barrier.
 15. Amethod of forming a magnetic tunnel junction (MTJ) stack of layersincluding a tunnel barrier layer between two magnetic layers,comprising: (a) providing a bottom magnetic layer with perpendicularmagnetic anisotropy (PMA); (b) depositing a first metal layer that formsa bottom magnetic layer/first metal layer interface; (c) performing afirst passive metal nitride deposition process with a maximum nitrogenpressure of about 10⁻⁵ torr, the first passive metal nitride depositionprocess deposits a first metal nitride layer on the first metal layerwhile keeping the bottom magnetic layer/first metal layer interface fromreacting with nitrogen; (d) forming one or more metal, metal oxide,metal oxynitride, or metal nitride layers on the first metal nitridelayer, steps (b)-(d) form a tunnel barrier layer; and (e) depositing atop magnetic layer on a top surface of the tunnel barrier layer.
 16. Themethod of claim 15 wherein the first metal layer has a thickness fromabout 1 to 6 Angstroms.
 17. The method of claim 15 wherein the firstmetal layer, the first metal nitride layer, and the one or more metal,metal oxide, metal oxynitride, or metal nitride layers comprise a metalselected from Mg, Al, Ta, Ti, Zn, Sn, MgZn, AITi, CoMg, and MgTa. 18.The method of claim 13 wherein the first metal layer is comprised of adifferent metal than in the first metal nitride layer, and in the one ormore metal, metal oxide, metal oxynitride, or metal nitride layers. 19.A method of forming a spin torque oscillator (STO) device, comprising:(a) forming a first metal oxide layer on a substrate that includes thesteps of: (1) forming a first metal layer on the substrate and oxidizingan upper portion thereof with a first passive oxidation having a maximumoxygen pressure of about 10⁻⁵ torr; and (2) forming one or more metal ormetal oxide layers on the oxidized upper portion of the first metallayer; (b) forming a spin polarization (SP) layer on a top surface ofthe first metal oxide layer; (c) forming a non-magnetic layer on the SPlayer; (d) forming an oscillation layer (OL) on the non-magnetic layer;and (e) forming a second metal oxide layer on the OL with a processcomprising: (1) forming a second metal layer on the OL and oxidizing anupper portion thereof with a second passive oxidation process having amaximum oxygen pressure of 10⁻⁵ torr; and (2) forming one or more metalor metal oxide layers on the oxidized upper portion of the second metallayer.
 20. The method of claim 19 wherein the first and second passiveoxidation processes have a maximum duration of about 1000 seconds, themaximum oxygen pressure is determined by directly controlling the oxygenpressure in a closed chamber, or by controlling an oxygen flow rate in avented chamber.
 21. The method of claim 19 wherein the first metal layerand the second metal layer each have a thickness from about 1 to 6Angstroms.
 22. The method of claim 19 where the one or more metal oxidelayers formed on the oxidized upper portion of the first metal layer andthe second metal layer are formed by one or more conventional methodscomprising: (a) direct deposition of a metal oxide layer; (b) depositinga metal layer and then oxidizing all or part of the metal layer with anoxygen pressure that is at least 10⁻³ torr; and (c) any combination orrepetition of steps (a) and (b) above.
 23. The method of claim 19wherein the first and second metal layers, and the one or more metal ormetal oxide layers formed on the oxidized upper portion of the firstmetal layer and the second metal layer are comprised of a metal selectedfrom Mg, Al, Ta, Ti, Zn, Sn, MgZn, AITi, CoMg, and MgTa.
 24. The methodof claim 19 wherein the first and second passive oxidation processes arefurther comprised of nitrogen.
 25. The method of claim 19 wherein atleast one of the one or more metal oxide layers formed on the topsurface of the oxidized portion of the first metal layer is furthercomprised of nitrogen and has a metal oxynitride composition.
 26. Themethod of claim 19 further comprising an anneal process following theformation of the second metal oxide layer, the anneal process comprisesa temperature up to about 450° C. for a duration up to about 90 minutes.27. The method of claim 19 wherein the first metal layer is comprised ofa different metal than in the second metal layer or in the one or moremetal or metal oxide layers formed on an oxidized upper portion of thefirst metal layer.
 28. The method of claim 19 wherein the second metallayer is comprised of a different metal than in the first metal layer orin the one or more metal or metal oxide layers formed on an oxidizedupper portion of the second metal layer.
 29. A method of forming an RFsignal generation device, comprising: (a) forming a spin torqueoscillator (STO) with a top surface and having at least one magneticreference layer (MRL) that contacts a first terminal, a magneticoscillation layer (MOL), and a first junction layer formed between theMRL and MOL; (b) forming a non-magnetic spacer layer on the MOL, thenon-magnetic spacer layer is connected to a second terminal; (c) forminga magnetoresistive (MR) sensor on the non-magnetic spacer, the MR sensorhas at least one magnetic sensing layer that is magnetostaticallycoupled with said MOL, a second magnetic reference layer, and a secondjunction layer that is a metal oxide formed between the magnetic sensinglayer and the second magnetic reference layer, the metal oxide is formedby a process comprising: (1) depositing a first metal layer on themagnetic sensing layer; (2) oxidizing an upper portion of the firstmetal layer with a passive oxidation process having a maximum oxygenpressure of 10⁻⁵ torr; and (3) forming one or more metal or metal oxidelayers on an oxidized upper portion of the first metal layer; and (d)forming a third terminal on the MR sensor, the magnetic sensing layerhas an oscillation state with an oscillation frequency that is inducedin said MOL when a magnetic field is applied to said STO and MR sensorin a direction perpendicular to the STO top surface concurrently with afirst electric current flowing between the first and second terminals,and the magnetostatic coupling generates magnetic oscillation with an RFfrequency in the magnetic sensing layer that produces a varying voltageacross the MR sensor when a second electric current flows between thesecond and third terminals.
 30. The method of claim 29 wherein thepassive oxidation process has a maximum duration of about 1000 seconds,and the maximum oxygen pressure is controlled by directly controllingthe oxygen pressure in a closed chamber, or by controlling an oxygenflow rate in a vented chamber.
 31. The method of claim 29 wherein thefirst metal layer has a thickness from about 1 to 6 Angstroms.
 32. Themethod of claim 29 where the one or more metal oxide layers formed onthe oxidized upper portion of the first metal layer is formed by one ormore conventional methods comprising: (a) direct deposition of a metaloxide layer; (b) depositing a metal layer and then oxidizing all or partof the metal layer with an oxygen pressure that is at least 10⁻³ torr;and (c) any combination or repetition of steps (a) and (b) above. 33.The method of claim 29 wherein the first metal layer, and the one ormore metal or metal oxide layers formed on the oxidized upper portion ofthe first metal layer are comprised of a metal selected from Mg, Al, Ta,Ti, Zn, Sn, MgZn, AITi, CoMg, and MgTa.
 34. The method of claim 29wherein the passive oxidation process is further comprised of nitrogen.35. The method of claim 29 wherein at least one of the one or more metaloxide layers formed on the top surface of the oxidized portion of thefirst metal layer is further comprised of nitrogen and has a metaloxynitride composition.
 36. The method of claim 29 further comprising ananneal process following formation of the MR sensor, the anneal processcomprises a temperature up to about 450° C. for a duration up to about90 minutes.
 37. The method of claim 29 wherein the first metal layer iscomprised of a different metal than in the one or more metal or metaloxide layers formed on an oxidized upper portion of the first metallayer.
 38. A method of forming a three terminal device, comprising: (a)forming a bottom magnetic layer as a polarizing layer that contacts afirst terminal; (b) forming a non-magnetic metal layer or a low RAtunnel barrier on the polarizing layer, where the low RA tunnel barrieris formed by a process comprising: (1) depositing a first metal layer onthe bottom magnetic layer; (2) oxidizing an upper portion of the firstmetal layer with a passive oxidation process having a maximum oxygenpressure of 10⁻⁵ torr; and (3) forming one or more metal or metal oxidelayers on the oxidized upper portion of the first metal layer; (c)depositing a middle magnetic layer as a free layer on the non-magneticmetal layer or the low RA tunnel barrier, the free layer contacts asecond terminal, a current flowing between the first terminal and secondterminal is used during a write operation; (d) forming a tunnel barrieron the free layer where the tunnel barrier formation process comprises:(1) depositing a second metal layer on the free layer; (2) oxidizing anupper portion of the second metal layer with a passive oxidation processhaving a maximum oxygen pressure of 10⁻⁵ torr; and (3) forming one ormore metal or metal oxide layers on an oxidized upper portion of thesecond metal layer; and (e) depositing a top magnetic layer as areference layer on the tunnel barrier, the reference layer contacts athird terminal, a current flowing between the second terminal and thirdterminal is employed during a read operation.
 39. A method of forming athree terminal device, comprising: (a) forming a bottom magnetic layeras a reference layer that contacts a first terminal; (b) forming atunnel barrier on the reference layer where the tunnel barrier formationprocess comprises: (1) depositing a first metal layer on the referencelayer; (2) oxidizing an upper portion of the first metal layer with apassive oxidation process having a maximum oxygen pressure of 10⁻⁵ torr;and (3) forming one or more metal or metal oxide layers on an oxidizedupper portion of the first metal layer; (c) depositing a middle magneticlayer as a free layer on the tunnel barrier, the free layer contacts asecond terminal, a current flowing between the first terminal and secondterminal is used during a read operation; (d) forming a non-magneticmetal layer or a low RA tunnel barrier on the free layer, where the lowRA tunnel barrier is formed by a process comprising: (1) depositing asecond metal layer on the free magnetic layer; (2) oxidizing an upperportion of the second metal layer with a passive oxidation processhaving a maximum oxygen pressure of 10⁻⁵ torr; and (3) forming one ormore metal or metal oxide layers on the oxidized upper portion of thesecond metal layer; and (e) depositing a top magnetic layer as apolarizing layer on the non-magnetic metal layer or low RA barrier, thepolarizing layer contacts a third terminal, a current flowing betweenthe second terminal and third terminal is employed during a writeoperation.